The purpose of this study is to determine whether mothers of twins, who have a higher level of gonadotropins that other mothers, have cancer incidence or cancer mortality rates different from those of controls. The study will be based on a follow-up of the experience of a study group of 3982 mothers who had male-female twins or other polyzygous births in Connecticut during the years 1925-1959; a control group of 3982 women matched pairwise on year of childbirth, age, number of previous children, race and national origin; and a second control group of 3982 women randomly selected from mothers of singletons. In addition, the Connecticut Twin Registry will be searched for male-female twins and other polyzygous births occurring during 1960-1974; we expect to identify mothers of 2500-3000 male-female twins from this search. The Vital Statistics Records will be searched for controls chosen in the ratio of 4:1, and matched on age, race, and index year of birth. These women will be added to the study and the Connecticut Tumor Registry searched for the women in the study and control groups recorded for since 1966 for cancers of all sites. The observed incidence of site-specific cancer will be compared with that expected among Connecticut residents of the same age and sex.